This invention relates generally to the field of valves and deals more specifically with positively retained valve seats which find particular utility in expanding type gate valves.
Expanding gate valves usually have fixed valve seats which are pressed in place in their seat pockets. Fixed seats of this type are not able to effectively compensate for lack of parallelism in the surfaces of the gate mechanism and seats, or for the other irregularities and imperfections that are inevitably present in the valve components. Moreover, pressed in seats are not positively held in place and tend to work their way out of the seat pockets, especially after the valve has been cycled repeatedly. Another drawback associated with the pressed in seat is the close tolerances that are required if the seat is to fit properly in place when the valve is assembled. A large percentage of seats fall outside of the acceptable tolerances and must be scrapped or reworked, thereby significantly increasing the cost of the valve. Pressed in seats are also difficult to replace in the field and to repair or recondition after prolonged use.
As an alternative to seats which are pressed in place, valve seats which are mounted for "floating" or limited floating movement toward and away from the gate have been proposed. This type of seat compensates for lack of parallelism and other irregularities in a generally satisfactory manner, although the added complexity of the valve body and seats caused problems with respect to economical manufacture and assembly of the valve. Furthermore, floating seats require a large cavity in the valve body in order to accommodate the various types of inserts that are used to limit the floating movement, and the size and weight of the valve are increased accordingly.